At present, reduction of the cost for crystalline silicon is one of the targets aspired after in the photovoltaic industry, where the competition is increasingly fierce. To lower the silicon raw material cost generally needs a development towards the direction of thinner silicon wafers, that is: using thinner silicon wafers is one of the future developing trends of crystal silicon solar cell industry.
When the minority carrier diffusion length is greater than the thickness of silicon wafer, the composite velocity on cells' both surfaces is becoming more and more important. To improve the quality of surface passivation and to reduce the surface composite velocity has become one of the main means to improve the efficiency of the cells. The aluminum back electrode can effectively reduce the back surface composite velocity and improve the conversion efficiency, which is a major back surface passivation structure generally used for commercial crystalline silicon solar cells.
Compared with the ordinary cells, the back passivated cells have passivated film on the surface to reduce the carrier composite and to improve the conversion efficiency; also it can improve the contact to reduce the contact resistance and increase the shunt resistance; in addition, it also has an effect of reduction of reflection.
When a passivation film is formed on the cell back surface, in order to allow the aluminum paste printing on the back electrode for better ohmic contact with solar cell substrate and better collection of electric current, a preparation process becomes available: {circle around (1)} the laser drilling method, the DuPont USA has developed a few kinds of paste incorporating this technology. ™Solamet® PV36x series new aluminum paste technology applied to the Rear-side Passivation of crystal silicon photovoltaic cell has successfully achieved 20.3% for conversion efficiency in cells. Higher conversion efficiency represents more power output. However, this technology requires an additional set of equipment plus complex processing, which would result in high cost. {circle around (2)} Chemical etching method, mainly using silicon paste on the composite passivated film to screen-print the silicon paste film with dot-contact pattern before etching by chemical liquid to remove the portions not covered by silicon paste film on the composite passivated film. MOK GmbH of Germany supplies the corresponding etching reagent. However, the disadvantage of chemical etching is that it can cause pollution.
The screen printing method for preparing strip-shaped local contact and back dot contact reveals that in the normal sintering condition, the aluminum paste is difficult to penetrate the silicon nitride film, and the aluminum alloy paste as formed therefrom is very poor with respect to ohmic contact causing current leakage increase. Since the silicon nitride film contains great density of positive electric charge, the formation of the inversion film will reduce short-circuit current and filling factor, and the scanning of this cell structure for series and shunt resistance demonstrates a big difference from the standard value. In case of using the method of etching paste for preparation of back contact, the electric performance parameters are a little bit improved. These research results has a notable significance to the production process for the solar cells.
The patent numbered [CN102667961A] discloses a aluminum paste to form aluminium back electrode in PERC silicon solar cell and its preparation method, which is used in the production of PERC silicon solar cells to improve electric efficiency with a good effect on the binding between the sintered aluminum paste and the back perforated passivation film, and the PERC silicon solar cell produced using aluminum paste under this invention has a longer service life and durability. However, there is still an small escape of Al—Si alloy from the perforation on the silicon wafer passivation film of the back surface during the sintering process, where the aluminum paste is printed on the back side with a perforated medium passivation film.
The Chinese patent numbered [CN101720512B] discloses a method for back contact on local back electrode, which uses screen printed local back electrode to form a high quality back contact, therefore, this will involve a provision of a back surface passivation and formation of back or rear contact featured with optical limit. However, after the back surface passivation, a perforation is still needed by chemical or laser way before aluminum paste printing to obtain the back dot contact after sintering.
The Chinese patent numbered [CN101540350B] discloses a preparation method for crystalline silicon solar cell, which is relating a preparation process of a rear dot contact crystalline silicon solar cells, forming a composite passivation film by thermal oxidation of silicon dioxide and silicon nitride plating on the wafer back, and on the composite passivation film, the silicon paste screen printing is applied to print silicon paste film of dot contact pattern, then the chemical etching liquid is used to remove the portions not covered by silicon paste film on the composite passivation film, also an aluminum paste film is printed on the silicon wafer back by screen printing and a silicon plus aluminum alloy film is formed between the silicon paste film and the aluminum paste film by sintering, finally the sintering results in a local ohmic contact and local aluminum back electrode between aluminum paste film and silicon base of silicon wafer back side. The preparation process using this method reduces the preparation cost, and mass production is easily realized in this way, the crystalline silicon solar cell with prepared backside dot contact electrode may have good back ohmic contact and local aluminum back electrode, it would reduce the increase of the series resistance caused by dot contact to some extent, thus maintaining a good backside passivation effect and optical back emission.
The Chinese patent numbered [CN202585431U] discloses an optical cell structure with back passivation, relating to a photo-voltaic cell structure with back passivation: back passivation film covering the backside of the solar cell, slotting on the back passivation film with slots orthogonal to each other to constitute a grid on the back passivation film, and the aluminum paste covering the slots on the back passive film, but not entirely covered. The aluminum paste sintering results in a local back electrode structure. The result shows the benefits of: {circle around (1)} reduction of the cell's series resistance, improvement of the filling factor and the cell conversion efficiency; {circle around (2)} improvement of the cell warpage thus the cells are flatter.
To sum up, in view of the domestic and foreign customers' requirement for performance of photo-voltaic cell structure with back passivation, the following shall be included: {circle around (1)} The longer service life; {circle around (2)} higher conversion efficiency; {circle around (3)} lower contact resistance, high shunt resistance; {circle around (4)} good reduction of reflection. No patent relating with above requirements is reported.